How to use
- The first time, calibrate — hold a bank card against the screen, drag the slider until the dashed box matches it, and save.
- Choose your distance from the screen (measure it) and stay there.
- Cover your left eye and press Test right eye. Pick the direction the bars of the E point; if you can't tell, press Can't tell.
- When the right eye is done, test the left eye and read the results.
Options explained
- Snellen (20/20)
- The notation used in the US and UK. 20/20 is normal vision; 20/40 means you need to be at 20 ft to read what a typical eye reads at 40 ft.
- Decimal
- The same result as a decimal. 1.0 = 20/20, 0.5 = 20/40, 0.1 = 20/200.
- Test distance
- A wall chart is read at 5 m (about 16 ft). A shorter distance works too; the E is scaled so it covers the same angle as at 5 m.
Notations compared
| Snellen (20 ft) | Decimal | 5-point log scale | Height of E at 5 m |
|---|---|---|---|
| 20/200 | 0.1 | 4.0 | 72.7 mm |
| 20/100 | 0.2 | 4.3 | 36.4 mm |
| 20/63 | 0.3 | 4.5 | 23.0 mm |
| 20/50 | 0.4 | 4.6 | 18.3 mm |
| 20/40 | 0.5 | 4.7 | 14.5 mm |
| 20/32 | 0.6 | 4.8 | 11.5 mm |
| 20/25 | 0.8 | 4.9 | 9.2 mm |
| 20/20 | 1.0 | 5.0 | 7.27 mm |
| 20/16 | 1.2 | 5.1 | 5.8 mm |
| 20/12.5 | 1.5 | 5.2 | 4.6 mm |
| 20/10 | 2.0 | 5.3 | 3.6 mm |
At 20/20 the whole E covers 5 minutes of arc (1/12 of a degree), and each bar and gap is 1 minute. Each row down is 0.79 times the size of the one above (10 to the power −0.1), so neighbouring rows differ only a little but ten rows differ tenfold. The 5-point scale is the Chinese national standard (GB 11533). At 3 m, the E for any row is 3/5 of its size at 5 m.
Getting an accurate result
- Measure the distance. Use a tape measure from your eyes to the screen. A 10% error shifts the result by about half a row.
- One eye at a time. Cover the other eye gently with your palm — don’t press on it or peek between your fingers.
- Don’t guess. If you can’t tell, say so. Don’t squint or tilt your head. You pass a row by reading 3 out of up to 5.
- Normal brightness. Turn off night mode and blue-light filters, and avoid glare on the screen.
How the test runs
It starts at the largest row, one E per row, skipping a row each time you are right so it finds your level quickly. After the first miss it steps back two rows and checks each row properly — up to 5 Es, 3 correct to pass — until a row is failed. Your result is the last row you passed. Each E points in a random direction, never the same as the one before.
If your vision has dropped
Children should have their eyes checked every year. If one eye suddenly blurs, straight lines look bent or you see dark patches, see an eye doctor straight away. Looking at something far away every 20 minutes and, for children, plenty of time outdoors help with eye strain and short-sightedness.
You can also check for astigmatism with the astigmatism test and colour vision with the colour blind test.
Frequently asked questions
Why do I need to calibrate?
The same number of pixels is a very different size on different screens. Acuity is measured from the angle the E covers in your eye, so the E must be its true size in millimetres. You calibrate once, and the setting is shared with the online ruler.
Is 20/25 or 20/32 a problem?
Slightly below 20/20 can come from the screen, the light or tired eyes, so test again another day. If it stays low, or a child's vision is dropping, book an eye exam.
My result is better or worse than at the optician.
A screen differs from a printed chart in brightness and contrast, and distance and calibration add some error, so a row or two of difference is normal. Use this for checking yourself between visits; glasses prescriptions come from an eye exam.
Can I use a phone?
Yes. Stand the phone upright on a table or have someone hold it, and step back to the distance you chose. If you are too close for the screen to draw the smallest rows, the page tells you which row it can go down to.
Can it tell me my prescription?
No. Acuity (how small you can read) and prescription (what lens you need) are different things; two people at 20/40 can need very different lenses. Only a refraction test measures the prescription.
Privacy
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